Several security or location systems are available on the market which provide determination of the location of a person in response to a distress signal. However, none of the references provided all of the requirements for adaption to a secure building, such as a prison.
U.S. Pat. No. 4,494,119 issued Jan. 15, 1985 in the name of Wimbush discloses a distress radio location method and system which utilizes a plurality of receivers in order to determine the location of the transmitter based on signal strength of the transmitted signal. The receivers report the signal strength of the transmitted signal to a central station which in turn calculates the transmitter's location based on the measured signal strengths and the known locations of the receivers and dispatched a rescue unit thereto. The signals may be encoded to identify the transmitter.
U.S. Pat. No. 4,549,169, issued Oct. 22, 1985 in the name of Moura et al discloses a security system wherein a transmitter is located on the body of a person for continuously transmitting a signal wherein when the person falls into the water such transmission is stopped. A low battery condition is also transmitted by the transmitted signal. A cradle is used to recharge and synchronize the timer. The code is transmitted within a predetermined time slot which is associated with the particular unit. Therefore, the monitor, upon receiving no transmission during a particular time slot, is able to determine that such unit is not properly functioning and distress condition exist.
U.S. Pat No. 3,419,865, issued Dec. 31, 1968 in the name of Chisholm discloses a mobile emergency unit locating system wherein receivers are all synchronized to a common clock and are located on vehicles. Each vehicle or unit includes a pulse transmitter and clock to actuate a transmitter during a unique time slot. Receivers deliver information to a central comparator to determine difference to times of arrivals of the same pulse signal to compute the position of the transmitter. The receivers are periodically resynchronized by a master signal. The transmitter transmits groups of coded pulses.